1. Field of the Invention
A liquid crystal display (LCD) device has at least one substrate with a coated alignment layer, and more particularly, to an alignment layer for initial alignment of a liquid crystal in an LCD device.
2. Discussion of the Related Art
Ultra thin flat panel display devices have a display screen with a thickness of several centimeters. Among these, LCD devices have been widely used for notebook computers, monitors, spaceships, aircrafts, etc. owing to their advantageous features of low driving voltage, low power consumption, portability, and the like.
Generally, an LCD device includes a color filter substrate having color filter layers formed thereon, a thin film transistor substrate facing the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer formed between these substrates.
In such an LCD device, alignment of the liquid crystal layer is varied by applying a voltage to control transmittance of light, thereby allowing an image to be reproduced. Electrodes are thus formed on the thin film transistor substrate and/or the color filter substrate for application of the voltage such that a pixel electrode is located on the thin film transistor substrate, and a common electrode is located on the color filter substrate so as to generate a vertical electric field between the two substrates (for example, twisted nematic (TN) mode). Alternately, the pixel electrode and the common electrode are located parallel to each other on the thin film transistor substrate so as to generate a horizontal electric field (for example, in-plane switching (IPS) mode).
FIG. 1 shows an exploded perspective view illustrating a related art TN mode LCD device.
As shown in FIG. 1, a thin film transistor substrate 10 includes a gate line 12, a data line 14 crossing the gate line 12, a thin film transistor T formed on a crossing region of the gate line 12 and the data line 14, and a pixel electrode 16 connected to the thin film transistor T. A color filter substrate 20 includes a light-shielding layer (or black matrix) 22, R, G and B color filter layers 24 formed on the light shielding layer 22, and a common electrode 25 formed on the color filter substrate 20.
A vertical electric field is generated between the pixel electrode 16 on the thin film transistor substrate 10 and the common electrode 25 on the color filter substrate 20, thereby allowing alignment of liquid crystals to be controlled.
Both substrates 10 and 20, constructed as described above, are bonded to each other to form a single liquid crystal panel. At this time, a liquid crystal layer is formed between the substrates 10 and 20.
Meanwhile, if the liquid crystal layer randomly aligns between the substrates 10 and 20, it is difficult to achieve a consistent arrangement of molecules in the liquid crystal layer. Thus, although not shown in the drawings, an alignment layer for the initial alignment of liquid crystals is formed on the thin film transistor substrate 10 and/or the color filter substrate 20.
Examples of a method for forming an alignment layer for initial alignment of the liquid crystal include a rubbing alignment method and a photo-alignment method.
In the rubbing alignment method, after an organic polymer such as polyimide is thinly coated on a substrate, a rubbing roll wound with a rubbing cloth is rotated to rub the organic polymer, thereby arranging the organic polymer in a constant direction.
However, the rubbing alignment method has the following drawbacks.
First, when the arrangement of the rubbing cloth becomes disordered, problematic light leakage may occur. FIG. 2 shows a schematic perspective view illustrating a disordered arrangement of the rubbing cloth.
As described above, since the structure such as the thin film transistor, the color filter layer and the electrode layers are formed on the substrate, some portion 32a of the rubbing cloth 32 wound around the rubbing roll 30 can become disordered when the rubbing roll 30 rotates on the structure formed on the substrate 10 or 20 as shown in FIG. 2. When the arrangement of the rubbing cloth becomes disordered, the chains of the organic polymer in a region rubbed by the disordered rubbing cloth cannot be aligned, resulting in light leakage in that region due to non-uniform alignment of the liquid crystals.
Second, when the rubbing cloth fails to contact the substrate, the problem-causing light leakage may occur. FIG. 3 shows a schematic perspective view illustrating an alignment state of the liquid crystals when the rubbing cloth fails to contact the substrate.
As described above, the electrode layers such as the pixel electrode and common electrode are formed on the substrate. Thus, as shown in FIG. 3, a region A where the rubbing cloth 32 fails to contact the substrate due to a step on the substrate 10 is formed. In this case, the alignment of the liquid crystals is not uniform in the region A, thereby causing the problem of light leakage.
In particular, in the TN mode LCD device, since the pixel electrode and the common electrode are formed in pixel regions on different substrates, respectively, there may not be very many regions having the steps formed thereon. However, in the in-plane switching (IPS) mode LCD device, since the pixel electrode and the common electrode are repeatedly formed in parallel in pixel regions on the substrate, there are many regions having the step formed thereon, whereby the problem of light leakage becomes serious.
The aforementioned problems in the rubbing alignment method are caused by the mechanism for providing physical contact between the rubbing roll and the substrate.
Recently, in order to solve these problems of the rubbing alignment method, various studies have been conducted for providing a method for manufacturing an alignment layer that does not require physical contact. In particular, instead of using the rubbing alignment method, use of a photo-alignment method has been suggested. In the photo-alignment method, an alignment layer is produced by irradiating polarized ultraviolet (UV) rays onto a polymeric film. In order to align the liquid crystals, the alignment layer must have an anisotropic structure, which can be formed when the polymeric film anisotropically reacts with the polarized UV rays.
However, although the photo-alignment method may address the above-described problems related to the rubbing alignment method described above, the photo-alignment method has a serious problem in that the anchoring energy is low. More specifically, with the rubbing alignment method, since the side chains of the organic polymer are arranged in the constant direction as described above, and grooves are uniformly formed over the surface of the substrate by rubbing, the alignment of the liquid crystals is controlled by mechanical interaction between the grooves and the liquid crystals as well as by chemical interaction between the side chains and the liquid crystals. In contrast, in the photo-alignment method, the alignment of the liquid crystals is controlled only by the chemical interaction between the side chains and the liquid crystals caused only by the photoreaction, without forming grooves on the surface of the substrate. Accordingly, in comparison to the rubbing alignment method, the photo-alignment method provides a lower anchoring energy and causes a problem of afterimage.
Since the problem of afterimage caused by the photo-alignment method is serious to such an extent that the method cannot be applied to large-scale production lines, the rubbing alignment method has been used for large production lines in spite of the light leakage problems.
As LCD devices of a higher quality have been increasingly required, there is a need to developing a method of aligning the liquid crystals that can overcome or minimize the problems of the rubbing alignment method and the photo-alignment method according to the related art.